This invention relates to a hinge mechanism suitable for supporting a vehicle mounted sun visor. Sun visors for vehicles are usually provided with a support arm extending from the vehicle structure and various means have been proposed in the past for attaching the visor to the support arm in such a way that it is frictionally held so that it can be held in a stored postion, sometimes with a detent operative at this stored position, for movement to any one of a number of positions where it is effective to shade the eyes of the vehicle driver or passenger from sunlight.
Previously employed designs for such hinge mechanisms have included a sleeve member engaged upon a rod member with the required friction provided by a spring clip engaged around the sleeve member. Often a portion of the spring clip is formed to extend through the sleeve and engage the rod member, which is manufactured with a "flat" portion on its surface, to provide a detent at the sun visor's stored position. Examples of this type of hinge mechanism are disclosed in British Pat. No. 1 534 282 and published European Patent Application No. 0 053 529.
These known hinge designs suffer from two distinct disadvantages: firstly the hinge mechanisms require the manufacture and assembly of at least three, sometimes complex, components at a commensurately high cost, and secondly the finished hinges are prone to rattle as a result of the number of components employed and the necessity to form some of the components from metal.
In order to overcome the problems inherent in the aforementioned type of hinge mechanism, simplified two-piece sleeve and rod designs have been proposed, in which at least one of the components is formed from a resilient plastics material.
Examples of this latter type of hinge mechanism are proposed in British Pat. Nos. 1 395 689 and 1 422 368. The design proposed in GB No. 1 395 689 comprises a shaft or rod member formed with a flattened portion, and a sleeve or bearing member having a complementary non-circular cross section to provide a detent position for an associated sun visor. This design suffers from the disadvantages set out in GB No. 1 422 368. These may be summarised as follows: if the bearing material is made soft enough to allow the visor to be hinged without damage to its structure, the bearing becomes permanently stretched and distorted by use and thus no longer functions to provide the required friction or detent position.
The design proposed in GB No. 1 422 368 was intended to overcome the problems of the earlier two-piece design by incorporating a gap in the sleeve or bearing member of the hinge described in GB No. 1 395 689. However in use the plastics forming the sleeve or bearing member tend to flow under load and the gap opens up, thereby reducing the grip of the sleeve or bearing member upon the rod to below an acceptable level. Furthermore, the corner formed where the flat portion meets the curved portion of the rod catches in the gap as the rod is rotated, pulling the gap apart and thereby exacerbating the gap's tendency to open in use.
Both the aforementioned hinge designs employ a flexible sleeve member. In West German Pat. No. 2 432 585 a hinge is proposed having a rigid non-circular sleeve and a resilient rod member confined therein. A disadvantage of this form of hinge mechanism is that, in order to provide the required friction and detent the rod is shaped to flex as it is rotated in such a way that the same part of its structure is repeatedly stressed in the same direction throughout its working life. This leads to a weakening of the rod's resilience in use (due to material flow when the rod is formed from plastics materials) and hence a degradation in the hinge's performance. Indeed, this deficiency was recognised by the authors of DE No. 2 432 585 since, in one embodiment, they show the reinforcement of the rod by a separate, metal spring member.